The invention applies for example to the cooling of superconducting components of particle accelerators.
The pressures discussed here are absolute pressures.
The cooling of the superconducting components of particle accelerators requires the use of a fluid in equilibrium at low temperature and at low pressure, particularly helium, of which the vaporization provides the necessary heat transfers.
The refrigeration installations used in these applications comprise liquefaction units which, using helium gas at atmospheric pressure and ambient temperature, are able to supply liquid helium in equilibrium with its gas phase at temperatures of about 2K and at pressures of about 30 mbar.
The power dissipated by the superconducting components vaporizes helium, which must be recompressed in order to be recycled to the liquefaction unit, of which the inlet pressure is set at a value in the range of atmospheric pressure. The role of the compression line is to control its own inlet pressure and hence the liquid helium temperature.
At present, only compression lines with centrifugal compressors in series are suitable for compressing, at the desired compression ratio, a sufficient flow rate for obtaining medium or high refrigeration capacities. The centrifugal compressors are accordingly dimensioned to provide the desired compression ratio for the nominal mass flow rate of helium gas vaporized by the superconducting components operating at full capacity.
During startup, waiting periods, or the operation of the superconducting components at reduced capacity, the cooling requirements decrease, and this is accompanied by a commensurate reduction of the mass flow rate of helium gas vaporized and introduced into the compression line. This decrease in the mass flow rate is liable to cause stalling of the compressors, which must provide a constant compression ratio.
In the case of compressors operating at ambient temperature, the above problem is solved simply by adjusting the flow rate of each compressor. For this purpose, each compression stage is provided with a recycle line, which can be used to increase the mass flow rate of each compressor, and thereby to prevent its stalling.
However, this solution cannot be applied to cryogenic compressors mounted in series, because the solution whereby gas is recycled between each compression stage in order to adjust the operating point of each compressor would require intermediate coolings, would prove to be extremely complicated to implement, and would remove cooling capacity from the refrigerator.
Such cryogenic compressors are also subject to specific contingencies, which must be taken into account for their control.
Thus, given that this concerns a cryogenic system, the temperature of each of the compression stages is variable.
Furthermore, given that the flow treated by the compressors is sent to a refrigerator, the latter imposes a flow rate limitation, corresponding to the flow rate which it can accept.
Moreover, the control process is not time-dependent. In fact, the flow rate of gas to be treated results from the evaporation of a portion of liquid, for which the pressure is decreased, this flow rate being obtained by appropriately varying the suction pressure of the first compression stage.
Such a variation is linked to the various parameters of the liquid, on which the pumping is carried out, that is, in particular, the quantity, superheat state, or the residual power dissipated in this liquid.